Significance. The Acute Respiratory Distress Syndrome (ARDS) is responsible for significant morbidity and mortality. Aside from lung protective mechanical ventilation strategies and conservative fluid management, there are no specific therapies for ARDS. A hallmark of ARDS is injury to the alveolar epithelium, including cell death, primarily of alveolar type (AT) I cells, resulting in permeability and the influx of edma fluid, which in turn leads to refractory hypoxemia. This injury phase is followed by a reparative response during which surviving ATII cells spread onto the denuded basement membrane, proliferate, and transdifferentiate into ATI cells, thus restoring barrier integrity. Importantly, epithelial repair is a primary determinant of recovery from ARDS. Innovation. Historically, basic ARDS research has focused on the injury phase; unfortunately, therapeutic interventions designed to mitigate injury have generally been unsuccessful. Investigations into reparative mechanisms have explored ATII cell proliferation. In this proposal, we focus on ATII cell spreading onto the denuded basement membrane after the sloughing of ATI cells. This phenomenon has been recognized since the 1970s but has not been well studied in animal models. We define spreading as an increase in surface area resulting in increased coverage of the denuded basement membrane. We have developed a novel method to rigorously measure spreading of genetically-labeled ATII cells using stringent stereologic techniques. We will characterize ATII cell spreading after mild and severe injury in relationship to proliferation, transdifferentiation, and the restitution of barrier integrity. Hypothesis. Based on our preliminar data, we hypothesize that Hypoxia Inducible Factor (HIF) 1? promotes ATII cell spreading and the restitution of barrier integrity via CXCR4/SDF1 signaling. Research Plan. Aim 1 will test whether in limited lung injury, rapid, pre-mitotic ATII cell spreading is associated with barrier restoration; in addition, after severe injury, requiring proliferation to replace lost cells, whethr post-mitotic spreading correlates with barrier restoration. Aim 2 will determine whether HIF1? is critical for spreading. Aim 3 will test the hypothesis that CXCR/SDF1 signaling promotes and mediates the role of HIF1? in ATII cell spreading. We will use both gain- and loss-of-function techniques, including ATII cell specific gene deficient mice. Our primary readout will be morphometric measurement of spreading (area of alveolar surface covered) using stereologic techniques. We will directly link in vivo studies to mechanistic spreading assays in cultured primary ATII cells. Conclusion. Here, we study repair of the injured alveolar epithelium by ATII cell spreading with an emphasis on the restitution of epithelial barrier integrity, a highly cliniclly relevant outcome. These studies will identify novel therapeutic targets to accelerate epithelial repair in ARDS, ultimately promoting the resolution of pulmonary edema and clinical recovery.